Intact pigment epithelium from chick embryos has no appreciable pentose phosphate pathway activity. In contrast, cultured pigment epithelial cells have a significant amount of activity, indicating that cultured cells metabolize in a way very different from those in vivo. Intact retinas from chick embryos demonstrate a decline in activity of this pathway during development. Cultured retinal cells remain generally undifferentiated in tissue culture and resemble the blast cells from early stages.